1. Field of the Invention
The present invention relates to an amplifier with pass-through mode.
2. Description of the Related Art
An amplifier built into an input device or the like of a radio communication device typically has a function for switching to a high gain or to a low gain according to an electric field intensity of a received radio frequency (RF) signal. Specifically, the amplifier executes an amplification mode for amplifying the received RF signal with a normal amplification factor in the case where the radio communication device is used in a region (weak electric field region) in which the electric field intensity of the received RF signal is weak, while the amplifier executes a pass-through mode for amplifying the received RF signal with an amplification factor lower than the normal amplification factor in the case where the radio communication device is used in a region (intense electric field region) in which the electric field intensity of the received RF signal is high. The amplifier capable of switching between the amplification mode and the pass-through mode according to the electric field intensity of the received RF signal is referred to as an amplifier with pass-through mode.
Referring to FIG. 7, a RF signal switching circuit disclosed in Japanese Laid-Open Patent Application Publication No. 2002-261501 is shown. In a case where a television tuner 40 is used in the weak electric field region, the amplification mode is selected, and an operation mode is set so that a voltage VB1 (e.g., 5V) equal to a power supply voltage is output from an output end of a band decoder 36. That is, a first RF signal path 31 is active, while a second RF signal path 32 is inactive. More specifically, a RF signal with a low level is fed to a RF signal input terminal 34 and then fed to an amplification field effect transistor 311 via a first diode 312 in an ON-state. The amplification field effect transistor 311 amplifies the RF signal up to a predetermined level, and then feeds the amplified RF signal to a RF signal output terminal 35 via a second diode 313 in an ON-state. At this time, a signal transmission transistor 321 is OFF, and the RF signal is not transmitted to the RF signal output terminal 35 via the signal transmission transistor 321.